The present invention relates to a steel structure adapted for the reinforcement of elastomers such as rubber conveyor belts, rubber tyres, rubber hoses, rubber timing belts or timings in polyurethane. The steel reinforcement comprises one or more steel filaments.
The present invention also relates to a method of treating a steel filament so that the steel filament receives a spatial wave form.
Such steel structures are widely known in the art. Recent prior art documents have disclosed a tendency towards steel structures where the steel filaments present one or another type of waviness, i.e. where, in addition to the plastic deformation as a consequence of the possible twisting of the steel filaments, the steel filaments have another plastic deformation. This additional and other plastic deformation is conveniently a consequence of a preforming operation, and results in a wavy pattern on the steel filament.
In this way U.S. Pat. No. 5,020,312 (Kokokuxe2x80x94priority 1989) and U.S. Pat. No. 5,111,649 (Kokoku) disclose steel cord structures consisting of three to five steel filaments. At least one steel filament is provided with a so-called xe2x80x98crimpxe2x80x99: this is a zigzagged form with relatively sharp angles, the sharpness depending upon the formation tools. The crimp is a planar wave form and is formed by means of two toothed wheels. The holes created at the level of the angles promote penetration of elastomer into the steel cord structure.
Another wave form has been disclosed in EP-A-0 462 716 (Tokusenxe2x80x94priority 1990). According to this document, the steel cords have three to twenty-seven steel filaments, 25% to 67% of which have a particular helix or helicoidal form. The plastical helix deformation is carried out by means of rotating preforming pins. The purpose is to promote penetration of the elastomer into the steel cord structure without increasing the so-called part load elongation (PLE, for definition see below). These steel cords are marketed under the name SPACY(copyright) cord. An important drawback of this cord is that its manufacture is energy-consuming or inefficient or both. Indeed, if the pitch of the helix is taken smaller than the twist pitch, then the rotation speed of the preforming pins must be more than twice as high as the rotation speed of a down-stream double-twister.
Still another wave form has been disclosed in WO-A-95/16816 (Bekaertxe2x80x94priority 1993). According to this document, the steel structure comprises steel filaments and at least one steel filament has been polygonally preformed. This is a spatial wave form and is the result of a preforming device with varying radii of curvature. The steel structures are marketed under the name BETRUO(copyright).
It is an object of the present invention to provide still another wave form to steel filaments of steel structures.
It is another object of the present invention to provide a wave form to steel filaments where the wave form combines advantages of existing wave forms.
It is still another object of the present invention to provide a wave form which can take a lot of specific forms depending upon the choice of the parameters of the wave form.
It is yet another object of the present invention to provide a wave form, the manufacture of which does not necessitate energy-consuming tools.
It is also an object of the present invention to provide a steel structure with an oval transversal cross-section as a consequence of the wave form of some filaments, e.g. a core filament.
According to the invention, there is provided a steel structure adapted for the reinforcement of elastomers. The steel reinforcement comprises one or more steel elements. At least one of these steel elements is provided with a first crimp and a second crimp. The first crimp lies in a plane that is substantially different from the plane of the second crimp.
In this way a spatial wave form is obtained without using driven and energy-consuming preforming tools.
Another advantage of this steel structure is that a lot of wave forms become possible. Indeed, the first crimp has a first crimp pitch and a first crimp amplitude. The second crimp has a second crimp pitch and a second crimp amplitude. This means already four design parameters which each can be varied, independently of each other over a certain range.
The first crimp pitch may be equal to or different from the second crimp pitch. With equal crimp pitches circular or oval spatial helixes can be obtained. Different crimp pitches, however, lead to spatial forms different from helixes.
The first crimp amplitude may be equal to or different from the second crimp amplitude. A different crimp amplitude enables to obtain a spatial form with an oval transversal cross-section on condition that the filament which is provided with the first crimp and the second crimp is not rotated around its own axis in the final steel structure.
Still another parameter which can be varied is the angle between the two planes. It is preferable, however, that the planes differ as much from each other as possible: so the best choice is a maximum difference of about 90xc2x0.
The steel element of the steel structure according to the invention can be a steel filament, a bundle of non-twisted steel filaments or a steel strand of twisted steel filaments. The steel structure according to the invention may also be constituted by any combination hereof.
The steel structure may be an untwisted structure consisting of one or more steel filaments lying parallel adjacent to each other and bound by each other by means of another wrapping filament or by means of an adhesive that is compatible with the elastomer to be reinforced.
An alternative embodiment is that the steel filaments lie nearly parallel adjacent to each other, which can be obtained by twisting them with a very large twist pitch e.g. by passing them at a relatively high linear speed through a twisting apparatus rotating at a convenient or relatively low rotation speed.
The steel structure may also be a twisted structure with some or all of the composing filaments twisted in to a coherent structure.
At least one of the first crimp pitch and the second crimp pitch is preferably smaller than the twist pitch of the steel filament provided with the first and the second crimp.
Within the general group of twisted structures, a first application of the invention are nxc3x971 steel cords, i.e. cords essentially consisting of two to five steel filaments.
In a first embodiment, some but not all of these filaments are provided with the first and the second crimp in order to allow rubber penetration. An example is a 4xc3x970.28 cord with one or two filaments provided with the first and the second crimp. Such a cord is used in the breaker plies of a tyre.
In a second embodiment, all of the filaments are provided with the first and the second crimp in order to increase the elongation at break above 5% or more.
An example is a 5xc3x970.38 cord with the five filaments provided with the first and second crimp. An additional advantage is that the cord may be twisted with a relatively large twist pitch (14 mm to 20 mm) without decreasing substantially the elongation at break. Another example are 4xc3x970.22 and 5xc3x970.22 where all filaments are provided with the first and the second crimp. These high elongation cords are suitable for reinforcing tyres of a motor cycle (lying at nearly 0xc2x0 with respect to the equatorial plane of a motor cycle tyre).
A second application of the invention are the so-called/+m (+n) steel cords comprising a core of/core steel filaments and a layer of m steel filaments twisted around the core. Additionally, a second layer of n steel filaments may be twisted around the first layer of m filaments.
One or more core steel filaments may be provided with the first and the second crimp in order to:
a) increase the penetration of the elastomer into the core;
and/or to
b) obtain an oval transversal cross-section of the core, and as a consequence, an oval transversal cross-section of the whole cord; and/or to
c) prevent the core steel filaments from migrating out of the cord.
An example is a 1+6 constructior. where the single core filament is provided with a first crimp and a second crimp in order to enable rubber penetration and in order to increase the anchorage of the single core filament in the cord, i.e. to prevent core migration.
The first crimp amplitude may be greater than the second crimp amplitude so that an oval transversal cross-section is obtained.
Another example is a 3+8+13 construction, where the three core filaments are provided with a first crimp and a second crimp in order to allow rubber penetration to the centre between the three core filaments.
A similar application is the replacement of the core filaments of the strands in a 7xc3x977 construction by a 2xc3x971 or 3xc3x971 element where the two or the three filaments are provided with the first and the second crimp.
Still another example is replacing the well-known construction 3xc3x97d+9xc3x97d+15xc3x97d by a 5xc3x97d1+9d+15xc3x97d, where the filament diameter d1 of the core filaments is smaller than the diameter d of the other filaments. The core filaments are provided with the first and the second crimp. Rubber penetration and elongation are increased and the stiffness is decreased.
A third application of the invention is the so-called nxc3x971 compact cords comprising n steel filaments which have been twisted with each other in the same twist sense and with the same twist pitch. An example is a 3xc3x970.365 |9xc3x970.345 CC (CC=compact cord) where all the core filaments are provided with the first and the second crimp in order to provide rubber penetration and in order to prevent core migration.
Another example is a 12xc3x970.38 CC where all twelve filaments are provided with the first and the second crimp in order to obtain a high elongation. Such a cord can be used as the weft or warp element in a woven structure adapted to reinforce rubber conveyor belts.
A fourth application is the multi-strand steel cord, which is a steel cord comprising two or more strands and where each strand consists of two or more filaments. if such strands are twisted in the cord in the same sense as the filaments are twisted in the strand (the so-called Lang""s lay cords) a high elongation at break can be obtained. A condition hereto is that relatively small twist pitches are used.
According to the invention, however, if some or preferably all filaments are provided with the first crimp and the second crimp, then much larger twist pitches are possible without loosing any elongation at break, and thus cords are possible which can be manufactured in a more efficient way.
It is also possible, still according to the invention, to combine the existing small twist pitches with a first and second crimp applied to all steel filaments. This allows to obtain a still higher elongation at break. The unavoidable loss in tensile strength and breaking load can be compensated by using an addition strand as core. The filaments of this core strand can also be provided with the first and second crimp.
A fifth application is a multi-strand steel cord, e.g. for use as reinforcement of conveyor belts, where the strands as a whole are provided with a first crimp and a second crimp, e.g. in order to obtain a rubber penetration between the strands.
According to another aspect of the invention, there is provided a method of giving to a steel filament a spatial wave form. The method comprises the following steps:
(a) applying a first crimp to said steel filament, said first crimp lying in a first plane;
(b) applying a second crimp to said steel filament, said second is crimp lying in a second plane substantially different from said first plane.